I want to find the first item in a sorted vector that has a field less than some value x.
I need to supply a compare function that compares 'x' with the internal value in MyClass but I can't work out the function declaration.
Can't I simply overload '<' but how do I do this when the args are '&MyClass' and 'float' ?
float x;
std::vector< MyClass >::iterator last = std::upper_bound(myClass.begin(),myClass.end(),x);
What function did you pass to the sort algorithm? You should be able to use the same one for upper_bound and lower_bound.
The easiest way to make the comparison work is to create a dummy object with the key field set to your search value. Then the comparison will always be between like objects.
Edit: If for some reason you can't obtain a dummy object with the proper comparison value, then you can create a comparison functor. The functor can provide three overloads for operator() :
struct MyClassLessThan
{
bool operator() (const MyClass & left, const MyClass & right)
{
return left.key < right.key;
}
bool operator() (const MyClass & left, float right)
{
return left.key < right;
}
bool operator() (float left, const MyClass & right)
{
return left < right.key;
}
};
As you can see, that's the long way to go about it.
You can further improve Mark's solution by creating a static instance of MyClassLessThan in MyClass
class CMyClass
{
static struct _CompareFloatField
{
bool operator() (const MyClass & left, float right) //...
// ...
} CompareFloatField;
};
This way you can call lower_bound in the following way:
std::lower_bound(coll.begin(), coll.end(), target, CMyClass::CompareFloatField);
This makes it a bit more readable